[3D Printing News] Introduction to SLS 3D Printing Technology
In this introduction to SLS 3D printing , we will guide you through the basic principles of laser powder sintering technology. After reading this article, you will understand the basic mechanism of the SLS process, as well as its advantages and limitations. Before that, let's take a look at how it works: (Further reading: Beginner's Guide: Comparison and Principles of Common 3D Printing Technologies )
Table of contents
| What is SLS? |
| How does SLS work? |
| Features of SLS |
| Common SLS materials |
| Post-processing |
| Advantages and limitations of SLS |
| Rules of thumb |
What is SLS?
Selective laser sintering (SLS) is an additive manufacturing process belonging to the powder bed melting family. In SLS, laser light selectively sintersects polymer powder particles, fusing them together and building parts layer by layer. The materials used in SLS are particulate thermoplastic polymers.
SLS is used for prototyping and small-batch production of functional polymer parts because it offers a very high degree of design freedom, high precision, and produces parts with good, consistent mechanical properties, unlike FDM or SLA technologies. The full potential of this technology can only be realized when designers consider its key advantages and limitations. 
SLS printing process
How does SLS work?
The following is how the SLS manufacturing process works:
I. First, heat the powder box and printing area to just below the melting temperature of the polymer, and then use a blade to re-spread a thin layer of powder onto the printing platform.
II. Then, the CO2 laser light will scan the contour of the next layer and selectively sinter (fuse) the polymer powder particles together. The entire cross-section of the component is scanned, solidifying the block.
III. Once this layer is complete, the print platform moves down, and the blade re-covers the surface. This process is then repeated until the entire section is finished.
After printing, the parts are completely encapsulated in unsintered powder, and the powder container must be cooled down before it can be opened. This process can take a considerable amount of time (up to 12 hours). The parts are then cleaned with compressed air or other explosive media and prepared for use or further post-processing. Any remaining unsintered powder can be collected and reused (only 50% of SLA powder is recyclable). 
SLS printer schematic diagram
Features of SLS
Printer parameters
In SLS (Silicone Lamination), almost all industrial parameters are preset by the machine manufacturer. The preset layer thickness used is 100-120 microns. A key advantage of SLS is that it does not require a support structure. The unsintered powder provides all the necessary support for the part.
For this reason, SLS can create free geometric shapes that cannot be achieved by other manufacturing methods.
When using SLS printing, the total print volume is very important, especially for small-volume production.
A box of a given height takes roughly the same amount of time to print, regardless of the number of parts it contains.
Because the recoating step determines the total processing time (laser scanning happens very quickly), and the machine must cycle through each layer the same number of times.
Packing can affect delivery time for small orders, as operators often wait until the boxes are full before starting to print.
Interlayer adhesion
In SLS, the interlayer bonding strength is excellent.
This indicates that SLS-printed parts have nearly isotropic mechanical properties. Standard polyamide powder (PA 12 or Nylon 12) is used.
The mechanical properties of the printed SLS samples (the most commonly used material in SLS) are shown in the table below and compared with the properties of the bulk nylon.
| XY direction | Z direction | Bulk PA12 | |
| Tensile strength | 48 MPa | 42 MPa | 35 - 55 MPa |
| Tensile modulus | 1650 MPa | 1650 MPa | 1270 - 2600 MPa |
| Elongation at break | 18% | 4% | 120 - 300% |
SLS parts have excellent tensile strength and modulus, comparable to bulk materials, but are more brittle (they have a higher elongation at break).
Because it is the internal cavity of the last part.
Typical SLS printed parts have a porosity of approximately 30%.
The porosity of SLS parts gives them a granular surface. This also means that SLS parts can absorb moisture, allowing them to easily absorb various colors in hot water.
However, special post-treatment is required if it is to be used in a humid environment. 
SLS parts available in a variety of colors. Their porosity makes them ideal for hot water dyeing.
Shrinkage and warping
SLS parts are prone to shrinkage and warping: as the new sintered layer cools, its size decreases and internal pressure accumulates, pulling the bottom layer upwards.
The typical shrinkage rate of SLS is 3% to 3.5%, which machine operators will take into account during the production preparation phase and adjust the corresponding design dimensions.
Large flat surfaces are most prone to bending. This problem can be solved by vertically orienting the part in the printing platform, but the best practice is to create a flat area with minimal thickness and design it to introduce cutouts to reduce volume. This approach reduces the overall cost of the part by using less material.
Over-sintering
Thermal radiation can cause unsintered powder around a feature to over-sinter during melting. This can lead to the loss of small details such as slots and holes.
Over-sintering depends on the size and wall thickness of the feature. For example, a slot 0.5 mm wide or a hole 1 mm in diameter can be successfully printed on a thin wall of 2 mm, but these features will disappear when the wall thickness is 4 mm or greater.
Based on experience, slots wider than 0.8 mm and holes with a diameter greater than 2 mm can be printed in SLS without worrying about over-sintering.
Remove powder
Because SLS does not require support material, it can easily and accurately print parts with hollow cross sections.
Because less material is used, hollow parts can reduce the weight and cost of the parts.
Pores are needed to remove unsintered powder from inside the part. It is recommended to add at least two pores with a diameter of not less than 5 mm in the design.
If high rigidity is required, the part must be fully printed and cured. Another approach is to create a design with holes to eliminate the need for pores.
This method involves embedding tightly packed powder into the part, increasing its mass and providing some extra support for mechanical loads without affecting printing time.
An internal honeycomb lattice structure can be added to the hollow interior (similar to the filled pattern used in FDM) to further increase the rigidity of the part. Hollowing out a portion in this way can also reduce warping. 
Removing powder from SLS parts
Common SLS materials
The most widely used SLS material is polyamide 12 (PA 12), also known as nylon 12. The price of PA 12 powder is approximately $50-60 per kilogram.
Other engineering thermoplastics, such as PA11 and PEEK, can also be used, but they are not as widely used.
Polyamide powder can be filled with various additives (such as carbon fiber, glass fiber, or aluminum) to improve the mechanical and thermal properties of SLS parts. Materials filled with additives are generally more brittle and may exhibit highly anisotropic behavior.
| Material | characteristic |
| Polyamide 12 (PA 12) | Good mechanical properties +Good chemical resistance - Matte, rough surface |
| Polyamide 11 (PA 11) | +Completely isotropic behavior +High elasticity |
| Aluminum-filled nylon (Alumide) | +Metallic exterior +High stiffness |
| Glass fiber filled nylon (PA-GF) | +High stiffness High wear resistance and high temperature resistance -Anisotropic behavior |
| Carbon fiber filled nylon (PA-FR) | + Excellent rigidity High weight-to-strength ratio -High anisotropy |
Post-processing
The powdery surface coating produced by SLS parts is easy to color.
By using various post-processing methods such as media polishing, dyeing, spraying, and painting, the appearance of SLS-printed parts can be improved to a fairly high standard.
Their functionality can also be enhanced by waterproof coatings or metal plating.
Advantages and limitations of SLS
The main advantages and disadvantages of this technology are summarized below:
+SLS parts have excellent isotropic mechanical properties, making them ideal for functional components and prototypes.
+SLS requires no support, so it can easily produce designs with complex geometries.
+SLS's manufacturing capabilities are well-suited for small to medium-volume production.
Currently, only industrial SLS systems are widely used, so delivery times are longer than other 3D printing technologies such as FDM and SLA.
SLS parts have a granular surface and internal porosity, and post-processing may be required if a smooth surface or water resistance is needed.
-SLS cannot accurately print large flat surfaces and small holes because it is prone to warping and over-clamping.
The following table summarizes the main features of SLA:
| Selective laser sintering (SLS) | |
| Material | Thermoplastic (usually nylon) |
| Dimensional accuracy | ±0.3% (lower limit ±0.3 mm) |
| Typical print size | 300 x 300 x 300 mm (up to 750 x 550 x 550 mm) |
| Normal layer thickness | 100 - 120 microns |
| support | unnecessary |
Rules of thumb
SLS can produce a large number of functional parts made of engineering plastics, most commonly nylon (PA12). The finished parts exhibit good mechanical properties and anisotropic motion. For components with special requirements, PA powder can be filled with additives.
References